1,623 research outputs found
A new measurement of the baryonic fraction using the sparse NGC 3258 group of galaxies
New X-ray observations of the sparse NGC 3258 group of galaxies made by the
ASCA satellite with good spectral and spatial resolution has revealed that this
group has a gravitational potential deep enough to prohibit significant mass
removal from the system. The baryonic fraction within 240 kpc is found to be
0.065 +0.051/-0.020 for h_{50}=1, where h_{50}=H_0/50 km/s/Mpc, in good
agreement with the universal value of 0.05 +/-0.01 predicted by standard Big
Bang nucleosynthesis for a Universe with Omega_0=1 and h_{50}=1. Since the deep
potential of the NGC 3258 group ensures that all pristine intragroup gas has
been retained, the baryonic fraction of the NGC 3258 group is indicative of the
universal value. Consequently it seems premature to rule out a critical
Universe.Comment: 19 pages Latex, using aasms4.sty, paper also available at
http://www.dsri.dk/~kristian To appear in Astrophysical Journal Letter
Evolution of the Luminosity Density in the Universe: Implications for the Nonzero Cosmological Constant
We show that evolution of the luminosity density of galaxies in the universe
provides a powerful test for the geometry of the universe. Using reasonable
galaxy evolution models of population synthesis which reproduce the colors of
local galaxies of various morphological types, we have calculated the
luminosity density of galaxies as a function of redshift . Comparison of the
result with recent measurements by the Canada-France Redshift Survey in three
wavebands of 2800{\AA}, 4400{\AA}, and 1 micron at z<1 indicates that the
\Lambda-dominated flat universe with \lambda_0 \sim 0.8 is favored, and the
lower limit on \lambda_0 yields 0.37 (99% C.L.) or 0.53 (95% C.L.) if
\Omega_0+\lambda_0=1. The Einstein-de Sitter universe with (\Omega_0,
\lambda_0)=(1, 0) and the low-density open universe with (0.2, 0) are however
ruled out with 99.86% C.L. and 98.6% C.L., respectively. The confidence levels
quoted apply unless the standard assumptions on galaxy evolution are
drastically violated. We have also calculated a global star formation rate in
the universe to be compared with the observed rate beyond z \sim 2. We find
from this comparison that spiral galaxies are formed from material accretion
over an extended period of a few Gyrs, while elliptical galaxies are formed
from initial star burst at z >~ 5 supplying enough amount of metals and
ionizing photons in the intergalactic medium.Comment: 11 pages including 3 figures, LaTeX, uses AASTeX. To Appear in ApJ
Letter
Telling the tale of the first stars
HE 0107-5240 is a star in more than once sense of the word. Chemically, it is
the most primitive object yet discovered, and it is at the centre of debate
about the origins of the first elements in the Universe.Comment: 3 pages, 0 figures, published in Nature "News and Views," Apr. 24,
200
Kinematics of Metal-Poor Stars in the Galaxy. II. Proper Motions for a Large Non-Kinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected without
kinematic bias, and with available radial velocities, distance estimates, and
metal abundances in the range 0.0 <= [Fe/H] <= -4.0. This update of the Beers
and Sommer-Larsen (1995) catalog includes newly-derived homogeneous photometric
distance estimates, revised radial velocities for a number of stars with
recently obtained high-resolution spectra, and refined metallicities for stars
originally identified in the HK objective-prism survey (which account for
nearly half of the catalog) based on a recent re-calibration. A subset of 1258
stars in this catalog have available proper motions, based on measurements
obtained with the Hipparcos astrometry satellite, or taken from the updated
Astrographic Catalogue (AC 2000; second epoch positions from either the Hubble
Space Telescope Guide Star Catalog or the Tycho Catalogue), the Yale/San Juan
Southern Proper Motion (SPM) Catalog 2.0, and the Lick Northern Proper Motion
(NPM1) Catalog. Our present catalog includes 388 RR Lyrae variables (182 of
which are newly added), 38 variables of other types, and 1680 non-variables,
with distances in the range 0.1 to 40 kpc.Comment: 31 pages, including 8 figures, to appear in AJ (June 2000), full
paper with all figures embedded available at
http://pluto.mtk.nao.ac.jp/people/chiba/preprint/halo4
Electrical transport properties of bulk MgB2 materials synthesized by the electrolysis on fused mixtures of MgCl2, NaCl, KCl and MgB2O4
Electrolysis was carried out on fused mixtures of MgCl2, NaCl, KCl and MgB2O4
under an Ar flow at 600C. Electrical resistivity measurements for the grown
deposits show an onset of superconducting transition at 37 K in the absence of
applied magnetic field. The resistivity decreases down to zero below 32 K. From
an applied-field dependence of resistivity, an upper critical field and a
coherence length were calculated to be 9.7 T and 5.9 nm at 0 K, respectively
The mass spectrum of metal-free Stars resulting from photodissociation feedback: A scenario for the formation of low-mass population III stars
The initial mass function (IMF) of metal-free stars that form in the initial
starburst of massive (virial temperatures >10^4K) metal-free protogalaxies is
studied. In particular, we focus on the effect of H2 photodissociation by
pre-existing stars on the fragmentation mass scale, presumedly determined by
the Jeans mass at the end of the initial free-fall phase, i.e., at the
so-called ``loitering phase,'' characterized by the local temperature minimum.
Photodissociation diminishes the Jeans mass at the loitering phase, thereby
reducing the fragmentation mass scale of primordial clouds. Thus, in a given
cloud, far ultraviolet (FUV) radiation from the first star, which is supposedly
very massive (about 10^3Msun), reduces the mass scale for subsequent
fragmentation. Through a series of similar processes the IMF for metal-free
stars is established. If FUV radiation exceeds a threshold level, the
star-forming clumps collapse solely through atomic cooling. Correspondingly,
the fragmentation scale drops discontinuously from a few time 10Msun to
sub-solar scales. In compact clouds (>1.6kpc for clouds of gas mass 10^8Msun),
this level of radiation field is attained, and sub-solar mass stars are formed
even in a metal-free environment. Consequently, the IMF becomes bi-modal, with
peaks at a few tenths of Msun and a few times 10Msun. The high-mass portion of
the IMF is found to be a very steep function of the stellar mass, xi_high(m)
being proportinal to m^{-5}. Therefore, the typical mass scale of metal-free
stars is significantly smaller than that of the very first stars. Also we study
the thermal instability in collapsing primordial prestellar clumps, and discuss
why the thermal instability occuring during the three-body H2 formation does
not appear to manifest itself in causing further fragmentation of such clumps.Comment: 34 pages, 6 figures, ApJ accepte
New limits on a cosmological constant from statistics of gravitational lensing
We present new limits on cosmological parameters from the statistics of
gravitational lensing, based on the recently revised knowledge of the
luminosity function and internal dynamics of E/S0 galaxies that are essential
in lensing high-redshift QSOs. We find that the lens models using updated
Schechter parameters for such galaxies, derived from the recent redshift
surveys combined with morphological classification, are found to give smaller
lensing probabilities than earlier calculated. Inconsistent adoption of these
parameters from a mixture of various galaxy surveys gives rise to systematic
biases in the results. We also show that less compact dwarf-type galaxies which
largely dominate the faint part of the Schechter-form luminosity function
contribute little to lensing probabilities, so that earlier lens models
overestimate incidents of small separation lenses. Applications of the lens
models to the existing lens surveys indicate that reproduction of both the
lensing probability of optical sources and the image separations of optical and
radio lenses is significantly improved in the revised lens models. The
likelihood analyses allow us to conclude that a flat universe with
Omega=0.3(+0.2-0.1) and Omega+Lambda=1 is most preferable, and a
matter-dominated flat universe with Lambda=0 is ruled out at 98 % confidence
level. These new limits are unaffected by inclusion of uncertainties in the
lens properties.Comment: 30 pages, 9 ps figures, AASTeX, ApJ in pres
Unavoidable Selection Effects in the Analysis of Faint Galaxies in the Hubble Deep Field: Probing the Cosmology and Merger History of Galaxies
(Abridged) We present a detailed analysis of the number count and photometric
redshift distribution of faint galaxies in the Hubble Deep Field (HDF), paying
a special attention to the selection effects including the cosmological dimming
of surface brightness of galaxies. We find a considerably different result from
previous studies ignoring the selection effects, and these effects should
therefore be taken into account in the analysis. We find that the model of pure
luminosity evolution (PLE) of galaxies in the Einstein-de Sitter (EdS) universe
predicts much smaller counts than those observed at faint magnitude limits by a
factor of more than 10, so that a very strong number evolution of galaxies with
\eta > 3-4 must be invoked to reproduce the I_{814} counts, when parametrized
as \phi^* \propto (1+z)^\eta. However we show that such a strong number
evolution under realistic merging processes of galaxies can not explain the
steep slope of the B_{450} and V_{606} counts, and it is seriously inconsistent
with their photometric redshift distribution. We find that these difficulties
still persist in an open universe with \Omega_0 > 0.2, but are resolved only
when we invoke a -dominated flat universe, after examining various
systematic uncertainties in modeling the formation and evolution of galaxies.
The present analysis revitalizes the practice of using faint number counts as
an important cosmological test, giving one of the arguments against the EdS
universe and suggests acceleration of the cosmic expansion by vacuum energy
density. While a modest number evolution of galaxies with \eta ~ 1 is still
necessary even in a Lambda-dominated universe, a stronger number evolution with
\eta > 1 is rejected from the HDF data, giving a strong constraint on the
merger history of galaxies.Comment: 24 pages, 15 figures, final version matching publication in ApJ. Some
references added. The complete ps file of Table 3 is available at
http://th.nao.ac.jp/~totani/images/paper/ty2000-table3.p
Hierarchical Formation of Galaxies with Dynamical Response to Supernova-Induced Gas removal
We reanalyze the formation and evolution of galaxies in the hierarchical
clustering scenario. Using a semi-analytic model (SAM) of galaxy formation
described in this paper, which we hereafter call the Mitaka model, we
extensively investigate the observed scaling relations of galaxies among
photometric, kinematic, structural and chemical characteristics. In such a
scenario, spheroidal galaxies are assumed to be formed by major merger and
subsequent starburst, in contrast to the traditional scenario of monolithic
cloud collapse. As a new ingredient of SAMs, we introduce the effects of
dynamical response to supernova-induced gas removal on size and velocity
dispersion, which play an important role on dwarf galaxy formation. In previous
theoretical studies of dwarf galaxies based on the monolithic cloud collapse
given by Yoshii & Arimoto and Dekel & Silk, the dynamical response was treated
in the extremes of a purely baryonic cloud and a baryonic cloud fully supported
by surrounding dark matter. To improve this simple treatment, in our previous
paper, we formulated the dynamical response in more realistic, intermediate
situations between the above extremes. While the effects of dynamical response
depend on the mass fraction of removed gas from a galaxy, how much amount of
the gas remains just after major merger depends on the star formation history.
A variety of star formation histories are generated through the Monte Carlo
realization of merging histories of dark halos, and it is found that our SAM
naturally makes a wide variety of dwarf galaxies and their dispersed
characteristics as observed. (Abridged)Comment: 24 pages including 29 figures, using emulateapj.cls; accepted for
publication in Ap
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